The role of optical spectral measurements for the monitoring of static and dynamic fluid systems is well established in the field of spectroscopy. Traditional systems involve the use of a spectrometric measurement system optically interfaced to a fluid stream. The standard format for such systems is some form of spectrometer or photometer (scanning) with some form of integrated sample handling system. In the case of spectrometer systems, commercial dispersive near-infrared (NIR) or FTIR (near- and mid-IR) instruments featuring some form of flow cell are good examples. Flow cells come in various forms for these types of applications, and can be used in transmission, transflectance (a combination of transmittance and reflectance) and internal reflectance formats. The internal reflectance format is often favored for traditional infrared measurements because it can be made to be minimally invasive and does not require a fixed film (fluid) thickness or optical pathlength. The latter can be constraining owing to the short pathlengths used in the mid-infrared region within a flowing fluid system. Furthermore, internal reflection probes are single sided and are easily inserted into either a flowing or static fluid system with no significant disruption to the fluid or the system being measured Examples of internal reflection probes are illustrated in U.S. Pat. No. 5,548,393 to Nozawa et al.
Optical spectroscopy, typically, but not exclusively, in the form of infrared spectroscopy is a recognized technique for the analysis and characterization of lubricants and functional fluids as used in the in the heavy equipment, automotive and transportation industries. Such spectroscopic measurements can provide meaningful data about the condition of the fluid and the fluid system during service. In the case of infrared spectroscopy, properties such as oxidation, coolant contamination, fuel dilution, soot, content, etc. can be derived by extraction of data from the spectrum. In most cases, this information is derived directly as a measure of the chemical functionality, as defined by the characteristic vibrational group frequencies observed in the infrared spectrum. Soot is a unique entity and is determined by a physical characteristic linked to light scattering and total light absorption (the nature of carbon particles).
While scanning systems are widely used for complex fluid systems, simpler, fixed wavelength, typically filter-based optical systems, are widely used for simple measurements on single-component and/or low-complexity fluid systems. For very simple systems, a single wavelength may be employed, thereby involving a very simple optical system comprising a source; often a polychromatic source used within a system that is optically constrained to a single measurement wavelength (frequency) by a suitable optical filter selected for the specific measurement. Such systems are used for the measurement of the following:                a) a single chemical entity (if a unique absorption, emission or fluorescence can be identified),        b) a phenomenon that involves optical attenuation, such as broadband absorption, turbidity or light scattering,        c) or in the case of calorimetric measurements, the measurement of a single color entity.        
More complex systems may be accommodated, and these require an increasing degree of optical complexity in terms of the ability to include additional analytical wavelengths to the measurement.
Traditionally, monitoring instruments are relatively large and expensive. If a simple, single-functional measurement is required it is possible to scale the device down in size. In essence, such a system involves only a light or energy source, a means for wavelength or energy selection, a means for interfacing with the sample and a means of detection. Such simple spectrometric/photometric systems can be made relatively small and compact.
Another factor to consider, however, is the operating environment. If a monitoring system is to be used in a relatively benign environment, such as in a laboratory or an ambient or conditioned indoor facility, then a traditional format of instrument may be used. If there is the requirement to measure a fluid system in a less conducive environment, such as on a process line (indoors or outdoors) or even on a vehicle or a mobile piece of equipment, then it is necessary to consider a system more ruggedly constructed than a traditional instrument. Standard enclosures are available that provide protection for instrumentation and these are commonly used for process monitoring applications. Such enclosures can include temperature and/or climate control and methods for vibration isolation.
The approach described above for transforming instrumentation into a format that is suitably prepared for monitoring fluids in “alien” environments (for traditional instrumentation) are expensive to fabricate. Furthermore, size can be a factor . . . once suitably packaged; even the simplest of measurement systems can become unnecessarily bulky or sometimes too large for convenient implementation. One option is to scale down the technology to the point where it becomes functionally equivalent to a sensor. For spectral measurement systems this is feasible. Examples exist for infrared measurements where combinations of micro-sources, optical filtration and small format detectors are combined to provide a practical monitoring system. The main problem with such devices is temperature sensitivity of the components and fragility in terms of long-term exposure to continuous vibrations. Also, intrinsically, like instruments, these devices are relatively expensive to fabricate.